Low dose-rate interstitial implants have been described as the treatment of choice "par excellence" for some types of human cancers. In recent years, 125I sources have been introduced for permanent implants as a substitute for more conventional radionuclides. The initial dose-rate in a typical implant is around 7 to 10 rad/hour and the isotope decays with a 60 day half-life. Tumor responses observed clinically are greater than might be expected from the very low dose-rates used. Mammalian cells in culture will be used to determine RBE values from the low energy photons (around 30 keV) emitted by 125I at the low dose-rates used clinically. Chemical sensitizers containing iodine, platinum, and possibly indium will be investigated; which are incorporated into or associated with the DNA. The K or L shell binding energies of these metal complexes are close to the energy of the photons emitted by 125I and so they may act as highly efficient absorbers leading to greatly increased local deposition of radiant energy. Before biological experiments are performed, extensive dosimetric measurements will be carried out because the currently used values for the output of 125I sealed sources are in doubt. Microdosimetric measurements will also be performed to form a basis for understanding the radiobiological data to be obtained.